This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-197192, filed Jul. 12, 1999; and No. 2000-073854, filed Mar. 16, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a color cathode-ray tube (CRT) apparatus, and more particularly to a color CRT apparatus capable of displaying a high-quality image, with reduction in oval deformation of a beam spot on a peripheral portion of a screen.
Self-convergence in-line type color CRT apparatuses, each having an electron gun structure with a BPF (Bi-Potential Focus) type DACandF (Dynamic Astigmatism Correction and Focus) system, have now been widely used.
The electron gun structure with the BPF type DACandF system, as shown in FIG. 16, comprises three cathodes K arranged in line; a first grid G1; a second grid G2; a third grid G3 having two segments G31 and G32; and a fourth grid G4. The grids G1 to G4 are disposed in the named order from the cathodes (K) side toward a phosphor screen. Each grid has three in-line electron beam passage holes which are formed in association with the three cathodes K.
A voltage obtained by superimposing video signals upon a voltage of about 150 V is applied to the cathodes K. The first grid G1 is grounded. A voltage of about 600 V is applied to the second grid G2. A DC voltage of about 6 kV is applied to the first segment G31 of the third grid G3. A dynamic voltage obtained by superimposing a parabolic AC voltage component, which increases in accordance with an increase in the degree of deflection of an electron beam, upon a DC voltage of about 6 kV, is applied to the second segment G32 of the third grid G3. A voltage of about 26 kV is applied to the fourth grid G4.
An electron beam generating unit is constituted by the cathodes K, first grid G1 and second grid G2. The electron beam generating unit generates electron beams and forms an object point for a main lens. A prefocus lens is constituted by the second grid G2 and the first segment G31 and it prefocuses the electron beams generated from the electron beam generating unit. A BPF type main lens is constituted by the second segment G32 and the fourth grid G4. The BPF type main lens accelerates the prefocused electron beams toward the phosphor screen and ultimately focuses them on the phosphor screen.
Where electron beams are deflected onto a corner portion of the phosphor screen, a potential difference between the second segment G32 and the fourth grid G4 takes a minimum value and the intensity of the main lens formed therebetween lowers to a minimum. At the same time, a maximum potential difference is provided between the first segment G31 and the second segment G32, and a quadrupole lens is formed which has a focusing function in a horizontal direction and a divergence function in a vertical direction. At this time, the intensity of the quadrupole lens takes a maximum value.
Where the electron beams are deflected onto a corner portion on the phosphor screen, a distance between the electron gun structure and the phosphor screen becomes longest and an image point is formed at a farther position. In the case of the electron gun structure with the above-described BPF type DACandF system, the formation of the image point at a farther position is compensated by decreasing the intensity of the main lens. In addition, a deflection aberration caused by a pin-cushion-shaped horizontal deflection magnetic field and a barrel-shaped vertical deflection magnetic field of a deflection yoke is compensated by the formation of a quadrupole lens.
In order to enhance the image quality in the color CRT apparatus, it is necessary to improve the focusing characteristics and beam spot shape on the phosphor screen. In the conventional in-line type color CRT apparatus, as shown in FIG. 17, a beam spot 1 formed on a central area of the phosphor screen is circular but a beam spot 1 formed on a peripheral area extending from an end of a horizontal axis (X-axis) to an end of a diagonal axis (D-axis) is deformed in an oval shape along a horizontal axis (X-axis) (xe2x80x9chorizontal deformationxe2x80x9d) due to deflection aberration and a blur 2 occurs along a vertical axis (Y-axis). The image quality is thus degraded.
In order to solve this problem, in the electron gun structure with the BPF type DACandF system, the low-voltage-side grid constituting the main lens is composed of a plurality of segments, like the third grid G3, and a quadrupole lens which has a lens intensity varying dynamically in accordance with a deflection amount of the electron beam is formed between the segments. Accordingly, the blur 2 of the beam spot 1 is eliminated, as shown in FIG. 18.
However, in the electron gun structure with the BPF type DACandF system, too, horizontal deformation occurs in the beam spot 1 formed on the peripheral area extending from the end of the horizontal axis (X-axis) to the end of the diagonal axis (D-axis), as shown in FIG. 18. The horizontal deformation of the beam spot 1 occurs because the electron gun structure is of the in-line type, the horizontal deflection magnetic field generated by the deflection yoke has a pin-cushion shape, and the vertical deflection magnetic field generated by the same has a barrel shape.
The horizontal deformation of the beam spot 1 will now be explained with reference to optical models shown in FIGS. 19A and 19B. In FIGS. 19A and 19B, an upper-side portion of a tube axis (Z-axis) corresponds to a cross-sectional view taken along a vertical axis (Y-axis), and a lower-side portion of the tube axis corresponds to a cross-sectional view taken along a horizontal axis (X-axis). FIG. 19A shows an optical model wherein an electron beam 4 is made incident on a central portion of a phosphor screen 5, without being deflected. FIG. 19B shows an optical model wherein the electron beam 4 is deflected and made incident on a peripheral portion of the phosphor screen 5. In these figures, ML denotes a main lens, QL denotes a quadrupole lens, and DL denotes a quadrupole lens component formed by deflection magnetic fields.
In general, the size of the beam spot 1 on the phosphor screen varies depending on a magnification M. The magnification M is expressed by a ratio of a divergence angle xcex10 of the electron beam 4 to an incidence angle xcex1i on the phosphor screen:
xcex10/xcex1i 
Where a horizontal divergence angle is xcex10h1, a horizontal incidence angle is xcex1ih1, a vertical divergence angle xcex10v1 and a vertical incidence angle xcex1iv1, a horizontal magnification Mh1 and a vertical magnification Mv1 are given by
Mh1=xcex10h1/xcex1ih1 
Mv1=xcex10v1/xcex1iv1 
Accordingly, where ≢0h1=xcex10v1, the following equation is obtained at the time of non-deflection, as shown in FIG. 19A, by the main lens ML having uniform focusing functions mainly in the horizontal and vertical directions:
xcex1ih1=xcex1iv1 
Therefore, Mh1=Mv1, and a circular beam spot is formed on a central portion of the phosphor screen.
On the other hand, at the time of deflection, as shown in FIG. 19B, in order to compensate the quadrupole lens component DL of the deflection fields having a diverging function in the horizontal direction and a focusing function in the vertical direction, the quadrupole lens QL having a focusing function in the horizontal direction and a diverging function in the vertical direction is formed in front of the main lens ML. Accordingly,
xcex1ih1 less than xcex1iv1 
and
Mh1 greater than Mv1 
Thus, an oval beam spot is formed on a peripheral portion of the phosphor screen.
As has been described above, in order to enhance the image quality of the color CRT apparatus, the focusing characteristics and beam spot shape on the phosphor screen need to be improved.
With the conventional electron gun structure of the BPF type DACandF system, a vertical blue of the beam spot due to deflection aberration is eliminated and the beams are focused over the entire area of the phosphor screen. However, in the case of the conventional electron gun structure of the BPF type DACandF system, horizontal deformation of the beam spot formed on a peripheral area extending from an end of the horizontal axis to an end of the diagonal axis on the phosphor screen cannot be eliminated. Consequently, the horizontal deformation of the beam spot interferes with the electron beam passage holes in the shadow mask, thus causing moire, etc. and degrading the quality of display images such as characters.
The present invention has been made in order to overcome the above problems, and the object of the invention is to provide a color cathode-ray tube capable of displaying a high-quality image, while reducing an oval deformation of a beam spot on a peripheral area of a screen.
According to the present invention, in order to achieve the above object, there is provided a color cathode-ray tube apparatus having an electron gun structure forming a plurality of electron lenses including a main lens for focusing electron beams on a phosphor screen, and a deflection yoke for horizontally and vertically deflecting the electron beams emitted from the electron gun structure,
wherein the electron gun structure comprises:
a focus electrode, an ultimate acceleration electrode and at least one intermediate electrode disposed between the focus electrode and the ultimate acceleration electrode, the focus electrode, the ultimate acceleration electrode and the at least one intermediate electrode forming the main lens;
voltage application means for applying to the focus electrode a dynamic voltage increasing in accordance with an increase in a degree of deflection of the electron beams, and applying to the intermediate electrode a voltage obtained by dividing a voltage applied to the ultimate acceleration electrode by means of a voltage dividing resistor; and
at least one additional electrode electrically insulatively covering a part of the electrode constituting the electron lens, the at least one additional electrode being electrically connected to the intermediate electrode.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.